The present invention relates to rigid polyurethane foam compositions, and more particularly, to rigid polyurethane foam compositions prepared with a mixture of lithium and zinc salts as catalysts.
The production of rigid polyurethane foams is a well known art, and such foams have a wide variety of industrial and commercial applications. Rigid polyurethane foams have been used as packaging materials, flotation materials and structural furniture components. One important use of rigid polyurethane foams is as insulating material. Rigid foams may be shaped into slabs or sheets of varying thicknesses and placed between walls and in roofs or in floors. Composites of metal-foam or wood-foam have been prepared by foaming the urethane material while in contact with the metal or wood. Such composites are useful as structural members for walls, floors and roofs. The rigid foams also may be formed into annular or contour shapes which are useful in insulating pipes and ducts. Rigid foams also can be directly applied to numerous substrates by spray foaming techniques. Spray foam applications are important particularly in such areas as warehouses, schools and offices to provide the desired insulation requirements for heating and cooling.
The catalysts which are utilized in the formation of polyurethane foams may differ depending on whether flexible or rigid urethane foams are produced. A wide variety of catalysts has been suggested for preparing flexible polyurethane foams, and these include metal as well as tertiary amine catalysts. See, for example, Polyurethanes: Chemistry and Technology Part II, Saunders and Frisch, Interscience Publishers 1964, pages 21-25 where the catalytic activity of various sodium, lead, iron, tin, copper, manganese, cadmium, etc. compounds, particularly salts, is discussed. The catalytic activity of typical nitrogen compounds also is reported. Studies of the activity of various catalysts for rigid foam systems are discussed in the same book at pages 217-219. Tertiary amines and tin compounds such as stannous octoate and dibutyl tin dilaurate are reported to be useful.
The use of various zinc salts as catalysts in various combinations has been described in the patent literature. For example, in British Pat. No. 980,139, zinc salts of aliphatic and aromatic carboxylic acids are described as useful in conjunction with known catalysts in the manufacture of foamed polyurethanes from polymers containing secondary hydroxyl end groups even though the zinc carboxylates themselves reportedly have little or no catalytic activity. U.S. Pat. No. 3,347,804 describes the preparation of polyurethane foams using a catalyst combination of tin, lead and zinc salts, particularly the carboxylates.
U.S. Pat. No. 3,884,849 describes low density, flexible and semi-flexible polyurethane foams which contain a small amount of solid chlorine-containing polymeric resin, zinc oxide and a zinc salt of an organic monocarboxylic acid. This zinc salt is added to the formulation prior to foaming and is added in addition to the normal catalysts. The zinc salt reduces or prevents discoloration of the urethane foam. Non-yellowing polyurethanes also are described in U.S. Pat. No. 3,580,873. In the procedure described in this patent, an isocyanate-terminated prepolymer is reacted with a mixture of monoethanolamine and triethanolamine in the presence of a catalytic amount of a metal drier compound such as calcium-, zinc-, or lead naphthenate or octoate.
Low-smoke generating polyurethane foams having incorporated therein a flame retardant and smoke inhibiting amount of a composition which comprises a halogen-containing organic compound and a metal salt of a polyfunctional aromatic carboxylic acid are described in U.S. Pat. No. 4,053,439. Zinc and lithium salts of aromatic carboxylic acids are included in the description.
Other patents which describe the use of various zinc salts of organic acids as catalysts for polyurethane formation include U.S. Pat. Nos. 3,751,392, 3,476,933, and German Offenlegungsschrift 2,435,217.
The use of alkali metal salts including lithium salts of organic acids as catalyst for polyurethane formation also has been described in the patent literature. Examples of patents include U.S. Pat. Nos. 2,894,919, 3,041,295, 3,108,975, 3,634,345, 3,769,245, 3,940,517 and 4,107,069. In U.S. Pat. No. 3,634,345, alkali metal salts of aromatic o-hydroxy carboxylic acids are described as being useful, either alone or in combination with other catalysts normally employed in isocyanate chemistry. U.S. Pat. No. 3,940,517 suggests catalytic combinations of alkali metal salts and other materials as long as the additional materials do not adversely affect the catalytic function of the composition. A catalytic composition comprising a tertiary amino phenol and potassium-2-ethyl hexoate is reported to give especially desirable results.
The above-described references represent a sampling of the suggestions which have been made of catalysts, particularly lithium and zinc catalysts, useful in the formation of polyurethane foams.
Lithium salts have not been widely accepted as catalyts because the lithium salts generally exhibit a severe threshold effect (high activity). It is difficult to obtain an acceptable foam profile with lithium salts. Zinc salts, however, often are not active enough to work in many foams, but when zinc salts are effective as catalysts in foam formation, the foam profile is good.